During the past forty years, it has become recognized that the use of organic compounds by society, and the disposal of those organic compounds, has had a deleterious effect on groundwater supplies. Contamination of groundwater is most commonly the result of spills and leaks from storage facilities or surface discharges of contaminants with the contaminates leaching into the groundwater over time. The most common types of contaminates are petroleum products and organic compounds incorporating various percentages of chlorine moieties on the molecule. In general, these compounds are referred to as volatile organic compounds (VOCs). The term VOCs is loosely used to include a wide range of organic contaminates which present a sufficiently high vapor pressure to allow various types of gas/liquid partitioning methods to be applied in the removal of the contaminates from an environmental system such as groundwater.
One prior art technique of clearing VOCs from groundwater has been the use of ex-situ air stripping of contaminates. In the ex-situ methodology, a contaminated environmental system such as groundwater, or a pond (above-ground water), is pumped to the ex-situ air stripping (EAS) facility where it is treated. The treated groundwater is then disposed of by known methods.
The basic idea in the operation of air stripping is that the substance, in this case, groundwater, containing the VOCs is sprayed into the air of a closed system. The spraying reduces the groundwater to small drops or particles, thus, significantly increasing the exposed surface area of the contaminated water. This increased surface area of the contaminated water increases the amount of water surface area that is in direct contact with the air of the surrounding environment. The increased surface area contact enables a greater amount of the VOCs contained in the water to be released from the water and to be absorbed into the adjacent air. This type of redistribution of a volatile compound is generally known as partitioning, or gas-liquid partitioning, of volatile compounds.
Gas-liquid partitioning is based on the fact that any volatile compound will redistribute itself between a liquid layer and gas layer to achieve an equilibrium state between the two layers. The equilibrium achieved is dictated by the particular physical properties of the volatile compound and the particular gas and particular liquid which make up the partitioning system. Since the redistribution between the gas and liquid phases is a constant factor basis based on the partition coefficient K of the system, the contaminated phase, in this case, groundwater, can be repeatedly exposed to air that contains less of the contaminate than the water, and by repeated exposure air, the contaminate can be drawn out of the water and the water contamination reduced to safer levels.
Typically, in an EAS operation, contaminated groundwater is pumped out of the ground to a treatment facility, treated and then discharged. The drawback of EAS water treatment is not in the effectiveness of the EAS process as air stripping towers can be designed which remove up to 99.9 percent of a contaminate from groundwater. The difficulty in EAS is the cost and success of extraction of the contaminated groundwater from the subsurface area. The extraction of the contaminates with the groundwater presents a difficulty as the contaminates within the groundwater can either be adsorbed under the soil, or the contaminates may exist in solution within the groundwater, or the contaminates may occur near the bottom of the groundwater layer as a result of the density of the contaminate being greater than the density of water. Another problem presented with simply extracting all groundwater containing the contaminates is that groundwater moves within the ground, and the contaminate suspended in the groundwater can become isolated in areas denominated as xe2x80x9cthe dead zone.xe2x80x9d The dead zone, in groundwater terminology, refers to pour spaces that are not connected with the general body of groundwater. Once contaminates have become located or isolated within pour spaces, or voids, in the soil, it is difficult to flush the contaminates out of these isolated pockets. Therefore, ex-situ air stripping of contaminated aquifers, or groundwater, may require thirty years or more of effort to achieve acceptable levels of contaminate reduction.
The second drawback associated with EAS is in the pumping of large volumes of groundwater out of the ground and to a treatment facility. Clearly, the movement of such large volumes of water over long periods of time attaches a very high cost to ex-situ air stripping of contaminates from groundwater. In view of the large volumes of water that must be pumped to the surface and treated, and adding to this cost the long lifetime of the project, the personnel costs over decades of treatment, monitoring and maintenance of the apparatus, such ex-situ air stripping projects can present unbearable costs.
An alternative method of groundwater remediation is known as air sparging. Air sparging is roughly similar to air stripping in concept in that the basis of operation is gas-liquid phase partitioning of the organic compound. In air sparging, air is bubbled under pressure through a contaminated aquifer. The air bubbles travel through the groundwater and adjacent soil during which time the organic compound is partitioned between the air and the liquid. The air bubbles then migrate to a vapor extraction area where the now contaminated air is removed. Typically, air sparging is operated on an in-situ basis by drilling multiple wells in the area of the contaminated groundwater, inserting air bubbling apparatus down into the well and forcing air or another gas into the contaminated aquifer. The injected gas rises through the water at the well site and is drawn off at the well head by a low-level vacuum.
In comparison to EAS, air sparging presents a number of advantages. In the use of air sparging, all groundwater treatment is performed in-situ, thus eliminating the pumping costs associated with EAS. Air sparging methodology treats the contaminates as they are found within the aquifer or groundwater thus eliminating the additional above-ground treatment facility apparatus. It will be appreciated that with EAS, wells must be initially drilled to extract the contaminated groundwater. The ex-situ nature of air sparging reduces operation and maintenance costs as the treatment facility is essentially located within the ground. Another advantage of air sparging is that it involves the injection of the oxygen of air into the subsurface water, thereby presenting opportunities for enhanced bio-degradation through oxidation as a result of the increased oxygen concentration within the groundwater.
An apparatus and method of removing contaminates from groundwater is provided which combines air stripping technology and air sparging technology into a single in-ground water well. The present invention combines the advantages of air sparging for the removal of VOCs with the advantages of air stripping while avoiding the high costs associated with the pump and treat methods of air stripping of contaminated groundwater.
The present invention provides for introduction of air or gas under sufficient pressure to enable release the gas into the well water pool and to allow the gas bubbles formed to travel upwardly through the well water pool. During this gas movement VOCs are partitioned between the liquid phase, in this case contaminated well water, and the gas phase which is the injected air or gas. The present invention further comprises the pumping of contaminated water from the well water pool to the atmosphere or area above the water pool at which point the contaminated water is sprayed into the well head area atmosphere to allow the sprayed droplets of contaminated well water to contact the well head air or gas. This permits the VOCs in the contaminated well water to become partitioned between the water droplet and the surrounding well head atmosphere. During use of the inventive method both the air sparging component and the air stripping component are applied and the resulting contaminated air or gas in the well head area is drawn off from the well head by a low pressure vent or vacuum vent that is attached to the well casing. The removed air or gas is collected and the VOC""s are removed from the air or gas stream.
The foregoing and other objects are intended to be illustrative of the invention and are not meant in a limiting sense. Many possible embodiments of the invention may be made and will be readily evident upon a study of the following specification and accompanying drawings comprising a part thereof. Various features and subcombinations of invention may be employed without reference to other features and subcombinations. Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, an embodiment of this invention.